WASM/PVM: refactor the output buffer

Context

This is a prerequisite for #4188 (closed)

• use options instead of negative values
• use message_info that wasn't used at all
• rename functions and variables to make their usage clearer
• comments

Manually testing the MR

We should trust the CI on this one and hope the tests are enough (I'll check it before assigning anyone for reviews).

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR